1. Field of the Invention
The present invention relates to a process for producing a photoelectric conversion film suitable, for example, for use in the reader unit of facsimile systems, and more particularly to a process for producing a photoelectric conversion film having improved image signal output characteristics and also improved photoresponse characteristics for realizing a real time image reading system.
2. Description of the Prior Art
Conventionally, photodetectors, such as CCDs and MOS sensors, which are fabricated by IC techniques have been used, for example, for the reader unit of facsimile systems.
However, such detectors are limited only to several tens of millimeters in length since they are prepared by IC techniques, so that in actual use, there is a need to form an image of the original on a reduced scale. To form images on a reduced scale, there arises a need to use a lens of substantial optical path length, and the detector must be positioned generally at a distance of 20 to 30 cm from the original. The great optical path length poses a serious problem in providing a compacted lightweight reader unit.
In contrast with the photodetector for use in such a scale reduction system, a contact-type image sensor has been proposed in recent years which has the same width as the document and which is provided with a fiber optic lens array thereon for forming an image of the original at a magnification of 1X.
While a CdS.sub.x Se.sub.1-x mixed-crystal film formed by vacuum evaporation, a Si film or the like is used for the photoelectric conversion portion of the image sensor, these films, which are prepared by a vacuum process, are costly and still remain to be improved in productivity, yield, etc.
On the other hand, a process for producing a photoelectric conversion film relatively inexpensively is known which comprises preparing slurry of finely ground cadmium sulfide crystals and/or finely ground cadmium selenide crystals, a small amount of activating impurities, flux and organic binder, coating a substrate with the mixture and baking the coated substrate in nitrogen gas or in a nitrogen gas atmosphere containing a trace (0.3%) of oxygen gas (for example, as disclosed in Examined Japanese patent publication SHO No. 52-25305).
Although this process affords photoelectric conversion films relatively inexpensively and with good reproducibility, the process is unable to prepare photoelectric conversion films which are outstanding in image signal output characteristics and in photoresponse characteristics for realizing real time image reading systems. In fact, it has been difficult to use the film obtained by the process for the facsimile reader unit which must handle a large number of picture elements as arranged with a high density.
On the other hand, a CdS.sub.x Se.sub.1-x film prepared from a solid solution of CdS and CdSe crystals in an optional ratio, for example, is used as the photoelectric conversion film of the contact-type image sensor. Nevertheless, the spectral sensitivity characteristics and photoresponse speed vary chiefly with this ratio. As the proportion of CdSe in the solid solution increases, the photoresponse speed increases, but the peak of spectral sensitivity shifts from 520 nm toward 720 nm off the center of the visible region. Pure CdSe has peak spectral sensitivity around 725 nm and is chiefly sensitive to red rays generally. Accordingly, when there is a need to read red character images, the film is not serviceable as a suitable photoelectric conversion element. In contrast, pure CdS is suitable for reading red character images but has the drawback of being lower than CdSe in photoresponse speed.
Thus there is a tendency for photoresponse to be in conflict with the spectral sensitivity characteristics, and there remains the problem that even if the solid solution has a suitable ratio, the photoelectric conversion film then available is always intermediate in respect to both the characteristics.
To solve the above problem, heat-treating methods are known for activating photoelectric conversion film. For example, an earthen cover is coated over the inner surface with an activating powder of CdS, Cd halide or the like in the form of a sintered coating, and the conversion film to be activated is heat-treated as enclosed with the cover. Examined Japanese patent publication SHO No. 58-46194 discloses another method wherein a powder prepared from a mixture of finely ground CdS and a Cd halide by calcining and pulverizing the mixture is placed on the bottom of a hermetic or semi-hermetic container, and the conversion film is heat-treated as placed within the container.
However, the former method, for example, has the problem that it is difficult to form a sintered coating of activating powder on the inner surface of the earthen cover, hence poor reproducibility.
The latter method requires a special hermetic or semi-hermetic container and has the problem that when the films to be activated are arranged in many stages within the container, spots occur on the surface of the films owing to an uneven flow of the activating vapor. The method has another problem in that a large amount of CdS or Cd halide needs to be used for each batch.